1. Field of the Invention
The invention relates to discharge lamp of the short arc type which is used as the light source of a projection device and a spotlight device.
2. Description of Related Art
Conventionally, a discharge lamp of the short arc type as shown in FIG. 5 is known as the light source of a projection device such as a projector or the like. In this discharge lamp of the short arc type, hermetically sealed tubes 2 of silica glass are formed bordering the two ends of a silica glass arc tube 1. Within the arc tube 1 there is a pair of tungsten electrodes 3 which are supported by tungsten lead pins 4 which are used to supply a high current to the electrodes 3. The lead pins 4 penetrate cylindrical retaining bodies 5 of silica glass which on the inside have one through opening each and which are cylindrical. These cylindrical retaining bodies 5 are mounted within the hermetically sealed tubes 2. The lead pins 4 are sealed by graded glass 6 in the hermetically sealed tubes 2. A vessel which is insulated against the outside is filled with a gas with a high pressure as the emission substance by this hermetically sealed arrangement.
In such a discharge lamp of the short arc type, since the internal pressure of the arc tube 1 during operation becomes very high in order to increase the radiance, a construction is necessary which the hermetically sealed tubes 2 are prevented from being damaged even at a high internal pressure. Furthermore, it is necessary for the lead pins 4 which support the electrodes 3 to project directly to the outside from the hermetically sealed tubes 2 which border the arc tubes 1 since a high current is flowing in the lamps. For hermetic sealing of the hermetically sealed tubes and the lead pins 4 to one another, therefore, a hermetically sealed arrangement using graded glass 6 is used.
The process for mounting the cylindrical retaining body 5 by the hermetically sealed tubes 2 is described below.
The inside of the arc tube and the hermetically sealed tubes 2 bordering the arc tube are initially subjected to a pressure reduction to form a negative pressure state. Upon heating the outside of the respective hermetically sealed tube 2 with a torch in which the respective cylindrical retaining body 5 is located, the diameter of the hermetically sealed tube 2 is reduced in the area that is heated. As a result, the hermetically sealed tube 2 and the cylindrical retaining body 5 are welded to each another. Thus, the cylindrical retaining body 5 is mounted within the hermetically sealed tube 2.
FIG. 6 shows a cross section of the hermetically sealed tube in a plane which perpendicularly intersects the lamp axis at the position at which the cylindrical retaining body is present.
As is shown in FIG. 6, there is a situation in which the silica glass cylindrical retaining body 5 and the tungsten lead pin 4, which penetrates the inside of the cylindrical retaining body 5, are temporarily welded to one another by heating when the hermetically sealed tube 2 is welded to the cylindrical retaining body 5. Between the inside of the cylindrical retaining body 5 and the outside of the lead pin 4, there is always an extremely small intermediate space S2, present as a gap that results from the different coefficients of thermal expansion of the materials. Therefore, the interior K1 of the arc tube 1 and the interiors K2 of the hermetically sealed tubes 2, as shown in FIG. 5, are continuously connected to each other.
Furthermore, there is an arrangement in which a metal foil is clamped between the inside of the cylindrical retaining body 5 and the outside of the lead pin 4 resulting in the respective part being completely insulated. FIG. 7 shows a cross section of a hermetically sealed tube in a plane which perpendicularly intersects the lamp axis at the position at which the cylindrical retaining body is present. Between the inside of the cylindrical retaining body and the outside of the lead pin, a metal foil is clamped.
As shown in FIG. 7, when the metal foil 8 is clamped between the inside of the cylindrical retaining body 5 and the outside of the lead pin 4, there is always an extremely small intermediate space S3 formed as a gap between the inside of the cylindrical retaining body 5 and the outside of the lead pin 4. As is shown in FIG. 5, the interior K1 of the arc tube 1 and the interior K2 of the hermetically sealed tube 2 are continuously connected to one another. See Japanese Patent document JP-A HEI 11-135067 and U.S. Pat. No. 6,356,018.
Japanese utility model application HEI 04-009963 (U.S. Pat. No. 5,200,669) discloses a construction in which there are intermediate spaces S2, S3 between the inside of the cylindrical retaining body 5 and the lead pin 4 which result in the following disadvantages.
As is shown in FIG. 6, the intermediate space S2 between the inside of the cylindrical retaining body 5 and the lead pin 4 is not a uniform gap around the lead pin 4, but instead the gap is eccentric with respect to the lead pin 4. In FIG. 6, the intermediate space S2 at the top of the page of drawings is larger than at the bottom.
As is shown in FIG. 7, in the case in which there is a metal foil 8 between the inside of the cylindrical retaining body 5 and the lead pin 4, the intermediate space S3 is also not in a uniform gap around the lead pin 4, but is instead a chaotic gap, due to the metal foil 8 which comes to rest on top of itself by winding around itself or by the effect of sagging of the metal foil 8.
During lamp operation of FIG. 5, due to a discharge phenomenon or due to the high temperature of the electrodes 3, a high temperature state is present in the interior K1 of the arc tube 1. Within interior K2 gas flow is ensured due to the extremely small gap between the cylindrical retaining body 5 and the lead pin 4. That is, the gas which is present in the interior K2 of each hermetically sealed tube 2 has a lower temperature than the gas present in the interior K1 of the arc tube 1.
This means that the gas present in the interior K1 of the arc tube 1 and the gas present in the interiors K2 of the hermetically sealed tubes 2 have a temperature difference during operation. The gas within the interior K2 of each hermetically sealed tube 2 passes through the intermediate spaces S2, S3 into the interior K1 of the arc tube.
This gas flow is formed by passage through the intermediate spaces S2, S3. However, as was described above, the intermediate spaces S2, S3 are not formed around the lead pin 4 in a uniform manner, but are formed in a non-uniform manner or chaotically. Therefore, the gas flow into the arc tube 1 does not take place symmetrically around the lead pin 4. As a result, the disadvantage occurs that the gas flow within the arc tube 1 is a non-uniform flow resulting in a fluctuating, a non-stabilized arc being formed. When the arc fluctuates, especially when the light source is in a projection device, there is the disadvantage that the flicker phenomenon occurs causing the image to become bright and/or dark.